1. Field of the Invention
The present invention relates to a diecasting apparatus and, more particularly, to a diecasting apparatus whose mechanism for supplying molten metal is improved so as to be suitable for diecasting of magnesium alloys.
2. Description of the Related Art
In a conventional diecasting apparatus comprising a supplying unit for supplying molten metal into a sleeve, and a mold unit having an inlet provided at a lower part thereof to which the sleeve is connected, a molten metal passage (a runner portion) extending vertically and a mold cavity communicating with the sleeve through the inlet and the molten metal passage, a diecasting operation is performed by: supplying molten metal into the sleeve; connecting the sleeve to the inlet at the bottom portion of the mold unit; and raising a plunger to send (inject) the molten metal stored in the sleeve into the mold cavity.
A ladle is used to pour molten metal into the sleeve after the sleeve is detached from the mold unit.
For an easier method of pouring molten metal with the ladle, the sleeve is tilted (e.g. as shown in FIG. 3 of Japanese Patent Publication No. 57-21414), or while being maintained vertically, the sleeve is moved away from the bottom of the mold unit (e.g. as shown in FIG. 4 of the above-mentioned patent specification).
FIG. 2 of the same patent specification shows a pouring method in which while the sleeve is connected to the lower mold, the lower mold is descended, and molten metal is poured from the ladle down into the descended lower mold through the molten metal passage.
In the above-described ladle-pouring method in which the sleeve is detached from the mold unit, a long interval is required between the pouring of molten metal into the sleeve and the injection of the molten metal into the mold cavity, and contamination is likely to occur. Further, in the pouring method in which the sleeve is tilted, the capacity of the sleeve cannot be fully utilized, so the sleeve must be made larger (longer) to compensate for the dead volume.
In the pouring method in which molten metal is poured down through the lower mold into the sleeve while the sleeve and the lower mold are kept connected, the injection of the molten metal poured in the sleeve can not be performed until the lower and upper molds are clamped. In other words, it takes a long time before the injection. Also, while being poured through the lower mold into the sleeve, the molten metal may spill on the top surface of the lower mold, i.e. the mating surface thereof.
In any of the methods described above, the molten metal is exposed to the atmosphere while poured into the sleeve, or even until the sleeve is connected to the inlet in such a method. The molten metal may well oxidize when exposed to the atmosphere. Thus, the above-described methods are unsuitable for diecasting of metals which easily oxidize (e.g., magnesium, aluminium and alloys thereof). Even in the case of diecasting of metals which do not so easily oxidize, an oxide may be produced or trapped therein to cause defects in the cast product.